Later date = better probability to find a PS3 with a CXD5300GGB RSX.
That said, there is still a part of luck, as someone got a 25XX 1A(=January/February/March 2011) with a CXD5300A1GB in it (the earliest RSX you can find in a 25XX, OCing less).
Sony getting rid of its old stocks, probably.
With the 28nm RSX there are only 2 VRAM chips (against 4 before), so I don't know.
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Nope. While good temperatures can help stabilizing, if an OC need that to be stable then you shouldn't keep it.
A good OC must be stable no matter the temps.